In a cellular mobile communications system, cell search is the procedure by which the user equipment (UE) achieves time and frequency synchronization with a cell and detects its cell ID. The UE is time synchronized when start of the symbols as well as the radio frame is found. Both symbol timing and frame timing need to be found for completing the cell search.
To improve the symbol timing performance, the synchronization signals are envisaged to be multiplexed several times per radio frame. For example, in WCDMA (Wideband Code Division Multiple Access), the synchronization channel (SCH) is transmitted 15 times per 10 ms radio frame. Thereby output statistics from the correlator performing the symbol timing acquisition can be accumulated, which improves the probability of correct symbol timing. Furthermore, to allow efficient handover between different radio systems, it is anticipated that the synchronization channel is multiplexed frequently and several times in a radio frame. However, a consequence of multiplexing the SCH signals several times within the frame is that frame timing does not follow directly from symbol timing. Mechanisms are therefore needed that, given the symbol timing, can determine the frame timing.
Two classes of SCH to be used in the cell search can be defined, a non-hierarchical SCH and a hierarchical SCH. The non-hierarchical SCH contains cell-specific signals that serve both for the complete timing- and frequency acquisition and cell ID detection. The hierarchical SCH consists of at least two signals; a known primary cell-common signal used only for symbol timing acquisition, and other cell-specific signals used for frame timing, frequency synchronization and cell ID detection.
A previously used concept for frame timing in hierarchical cell search, shown in reference documents [1], [2], and [3], which are completely identified in the list at the end of this specification, includes transmission of different signals in the SCH slots within a frame. (At the end of this specification there is a list of reference documents.) Given the symbol timing, the signals in the SCH slots are detected independently, but together they represent elements of a codeword from a synchronization code. Since the SCH is periodically transmitted, the receiver can detect any cyclic version of a codeword. The code must therefore be constructed so that all cyclic shifts of a codeword are unique and no codeword is a cyclic shift of another codeword. Thereby the frame timing can be uniquely determined from the cyclic shift of the detected codeword.
In WCDMA there are 512 scrambling codes (cell IDs), which are grouped into 64 scrambling code groups including 8 codes each. Each of the 64 code groups is represented by a codeword. For detecting the frame timing, the soft decision maximum likelihood principle includes computation of decoding metrics for the codewords and all their cyclic shifts. Thus an advantage of this hierarchical solution is that it reduces the search for frame timing to the decoding of 64 codewords, i.e. reduction to much less codewords than the number of cell IDs is done.
However, in a fully non-hierarchical SCH, it is foreseen that the cell ID and frame synchronization are detected only from the SCH signals within the frame, i.e. no use of hierarchical cell ID grouping or other channels should be needed. Correspondingly, in a non-hierarchical solution, one would need to decode and compute metrics for 512 codewords (cell IDs) and their cyclic shifts at once. Since the cell ID detection is done both initially for finding the home cell and continuously for supporting mobility by finding neighbor cells, such an exhaustive procedure may become overly tedious, use a lot of computing and power resources in the UE and prolong the cell search time. Moreover, as has been discussed for the E-UTRA system, not only cell IDs but also additional cell-specific information may be included in the cell search procedure, e.g. channel bandwidth, number of antennas, cyclic prefix lengths, etc. This would require even larger sets of codewords that need to be efficiently decoded.
Therefore, in particular for non-hierarchical SCHs and/or cases where a lot of cell-specific information should be included in the cell search, novel code designs and methods to detect the frame timing are required. It is desirable to give the codewords some form of structure that can be utilized by the receiver. There is a need for achieving performance close to the maximum likelihood decoder, while keeping low decoding complexity by employing some form of systematic decoding algorithm, tailored to the structure of the code.
A synchronization code should be designed that is able to carry cell-specific information and have a structure for frame timing synchronization. Several different synchronization signals are assumed to be multiplexed in to a radio frame, and the allocation of these signals to the frame (i.e. the codeword design) should be done so that it allows for efficient systematic decoding and performance comparable to maximum likelihood decoding.